Utils: Add CommandLine convenience constructors

... taking a QString for the executable.

This weakens the very explicit QString -> FileName conversion via the
named constructors for the special case of constructing a CommandLine.

I think that's worthwhile here, as it reduces the noise on the caller
site under circumstance where the nature of the thing is obvious.

Change-Id: I27b4a73639728893d053b2e7ba65cb745f0ffe83
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/git/gerrit/gerritmodel.cpp

@@ -314,8 +314,7 @@ void QueryContext::start()
     fp->setKeepOnFinish(Core::FutureProgress::HideOnFinish);
     m_progress.reportStarted();
     // Order: synchronous call to error handling if something goes wrong.
-    VcsOutputWindow::appendCommand(
-                m_process.workingDirectory(), {Utils::FilePath::fromString(m_binary), m_arguments});
+    VcsOutputWindow::appendCommand(m_process.workingDirectory(), {m_binary, m_arguments});
     m_timer.start();
     m_process.start(m_binary, m_arguments);
     m_process.closeWriteChannel();